1. Field of the Invention
The present invention relates to an automatic focus sensing device that automatically senses an in-focus point by driving an imaging lens.
2. Related Art Statement
In one of known autofocusing methods employed in image pickup apparatuses having an image pickup device, the contrast information of an object indicative of the degree of focus, such as a contrast value, is obtained based on an image signal that is acquired when a focusing lens or an imaging lens is projected or recessed. The position that offers the maximum value, namely, the maximum-contrast position, is regarded as the in-focus position, and the focusing lens is driven to that position. This autofocusing method is called the "mountain climbing" method, detailed in "NHK TECHNICAL JOURNAL" (86, Vol. 17, No. 1, 1965), page 25.
Japanese Unexamined Patent Publication No. 5-236326 discloses, as a fast driving method for driving a focusing lens in the above image pickup apparatus, a focusing method for driving a focusing lens in non-synchronization with a synchronization signal for generating a video signal. In this method, the driving of the focusing lens is suspended for a duration during which contrast information indicative of the degree of focus within an in-focus area is generated, and at the moment the contrast value is obtained, the amount of driving for the focusing lens is determined and the focusing lens is driven. In this way, the focusing lens is driven without waiting for the synchronization signal, thereby ensuring a fast focus control.
In another known method, focusing speed is increased by picking up contrast values at three positions of a focusing lens, and this method is disclosed in Japanese Unexamined Patent Publication 6-118297. According to this disclosure, contrast values are picked up at three positions with priority, namely at the center position within the position adjustment range of an imaging system and two other positions on both sides of the center position. If the center position presents a maximum contrast, this position is determined to be an in-focus position. If that position is not determined so, a determination is further made of whether either of both side positions presents a higher contrast value. While the point of measurement of contrast value is shifted to the higher contrast position side, a determination is made of whether the higher contrast position is in-focus depending on whether it gives a maximum contrast.
Conventionally available is a method that employs a plurality of AF (autofocus) areas for autofocus sensing to monitor concurrently contrast values within the respective AF areas and to switch to an optimum AF area. Such a method is disclosed in Japanese Unexamined Patent Publication 6-233171, for example. According to this method, the focusing operation is possible even if one AF area fails to provide its contrast value, as long as other AF areas successfully provide their contrast values.
In the known focusing method for driving a focusing lens in non-synchronization with a synchronization signal for generating a video signal, the focusing lens is suspended while the contrast information indicative of the degree of focus within the in-focus area is produced. In such a method, as the in-focus area expands, the time required to produce the contrast information is accordingly prolonged. For this reason, the time allowed to drive the focusing lens is shortened, and the focusing time until the completion of a focusing operation is thereby delayed.
Furthermore, in the known method where the focusing speed is increased by picking up contrast values at three positions of the focusing lens, the center position is determined to be an in-focus position if it presents a maximum contrast. This expedites the focusing operation. If that position is not determined so, a determination is further made of whether either of both remaining positions presents a higher contrast value. While the point of measurement of contrast value is shifted to the higher contrast position side, a determination is made of whether the higher contrast position is in-focus depending on whether it gives a maximum contrast. If the in-focus position fails to agree with the center position within the position adjustment range, this method simply performs the known "mountainclimbing" method sequentially and no substantial speedup of focusing operation is expected.
In the method that employs a plurality of AF areas for autofocus sensing to monitor concurrently contrast values within the respective AF areas and to switch to an optimum AF area, the following problem arises in a focusing operation. To achieve a faster focusing speed, the lens is driven even while an image pickup device accumulates and integrates the light from an object. Particularly when the AF areas are arranged vertically, resulting contrast values fail to agree in the in-focus position, and a proper focusing operation cannot be carried out.